Authenticate user named username with the one-time password otp and (optional) password passwd. Credentials are read (and updated) from a text file named usersfile.

Note that for TOTP the usersfile will only record the last OTP and use that to make sure more recent OTPs have not been seen yet when validating a new OTP. That logics relies on using the same search window for the same user.

On successful validation, OATH_OK is returned. If the supplied otp is the same as the last successfully authenticated one-time password, OATH_REPLAYED_OTP is returned and the timestamp of the last authentication is returned in last_otp. If the one-time password is not found in the indicated search window, OATH_INVALID_OTP is returned. Otherwise, an error code is returned.